To avoid inadvertent triggering of a safety device it has been proposed to utilize two units, namely an arming unit and a crash sensing unit, to produce, respectively, an arming signal and a crash signal, the safety device being triggered only when the arming signal and crash signal are present simultaneously. An arrangement of this type is intended to reduce the risk that a single mechanical or electrical error might lead to an inadvertent triggering of the safety device.
The arming unit may comprise a simple acceleration sensor which generates the arming signal if the acceleration exceeds a predetermined level. Alternatively the arming unit may include one or more sensors and a processing unit which performs a predetermined algorithm, the arming algorithm, on the signal or signals received from the one or more sensors to provide the appropriate output.
The crash sensing unit may again comprise a simple acceleration sensor which generates a signal if the acceleration exceeds a predetermined level, but again the crash sensing unit may comprise one or more sensors associated with a processor which is configured to perform an algorithm, the crash algorithm, on the signal or signals received from the one or more sensors to provide the appropriate output.
The crash sensing unit typically includes a sensor which is directionally sensitive, the sensitive direction of the accelerometer being aligned with the crash direction which is to be sensed by the sensor. The crash algorithm may be determined by whether or not the acceleration sensed by the accelerometer, acceleration “a” or its integrated value over time, Δv exceed a predetermined threshold.
It has been proposed to use the signal from a proximity sensor such as a radar configured to provide an output related to the distance to an object and/or the relative speed between the object and the sensor, to improve the ability to discriminate between different potential crashes, so as, for example, to discriminate between a potentially dangerous crash and a potentially not dangerous crash. If a doppler radar is utilized, an output can easily be provided which represents the relative velocity vrel between the sensor and a nearby obstacle. If a pulse radar is used, the time delay of the reflected pulse will provide an indication of the distance to the object. Repeated measurements of the distance at short time intervals can enable the relative velocity vrel to be calculated, for example by differentiation of the measured distance with respect to time.
It has been suggested, for example in U.S. Pat. No. 5,835,007, to utilize an anticipatory crash sensor of the radar type in conjunction with an accelerometer, with the threshold value of acceleration being calculated in dependence upon the signal from the anticipatory crash sensor.
It has been found that the use of a proximity sensor may not result in the required impact prediction. If an arrangement for triggering a vehicle safety device is arranged so that a signal from a proximity sensor is essential for the triggering signal to be generated, then should the proximity sensor fail to provide an appropriate signal (for example if it fails to “see” the object), the corresponding triggering signal will not be generated. This is clearly disadvantageous.
The present invention seeks to revise an improved arrangement for triggering a vehicle safety device.